The present invention relates to an automatic wire threading method for a wire-cut electric discharge machine, and more particularly, to a method of this type in which a wire electrode (hereinafter referred to as the wire) can be passed through a fine-diameter initial hole for machining using a simple arrangement.
A wire-cut electric discharge machine is provided with a device for automatically connecting a wire, which is artificially cut when one machining cycle is replaced by another or snapped during machining work, while the wire is being transported from the wire supply side of a wire transportation system to the wire delivery side thereof, so that the electric discharge machine is capable of automatic continuous operation. The performance of the electric discharge machine greatly depends on the probability of success in the automatic wire threading.
Conventionally, therefore, various methods and apparatuses for automatic wire threading have been proposed. Meanwhile, in precision machining, an initial hole for machining or a machining start hole, which allows the wire to be passed through a workpiece in the initial stage of the machining, has a very small diameter of, e.g., 0.8 mm or thereabout. In such a case, the probability of success in the automatic wire threading by means of the conventional methods and apparatuses is not always high, and the arrangement is relatively complicated.
An apparatus disclosed in Japanese Patent Disclosure No. 55-164437, for example, is constructed so that a receiving member is introduced beyond a wire guide and a workpiece when the wire is to be cut, and the receiving member is returned after the supply- and delivery-side ends of the wire are coupled by means of the member. This apparatus requires an introducing device for the receiving member and mounting means for connecting the introduced receiving member and the wire.
According to a method and an apparatus disclosed in Japanese Patent Disclosure No. 56-9134, moreover, that portion of a wire which corresponds to an electric discharge machining region is hardened by heating and straightened. After this wire is cut at a position below a workpiece, the straight portion is drawn out temporarily from the workpiece, the workpiece is horizontally moved, and the straight portion is then passed again through a machining start hole. This arrangement, therefore, requires means for ensuring the straightness of the wire.
In a method disclosed in Japanese Patent Disclosure No. 56-52135, moreover, a linear or rod-shaped guide is previously passed through a machining start hole in a workpiece, and the guide and the wire are connected when the wire is passed through another machining start hole. This arrangement requires the trouble of previously passing the guide through each machining start hole. It is difficult to apply this arrangement to automatic wire threading by means of a fine-diameter machining start hole, even though an electrode used for the formation of the machining start hole is employed directly as the guide.
In an apparatus disclosed in Japanese Patent Disclosure No. 56-82133, furthermore, a pipe for guiding a wire is inserted into a machining start hole of a workpiece so that it reaches a lower guide position, at the time of automatic wire threading. It is also difficult to apply this arrangement to the case using a fine-diameter machining start hole.
Further, there is an alternative arrangement in which a narrow water current with a diameter of about 1 to 2 mm is ejected from a sub-nozzle, in intimate contact with a guide, after a wire is cut in the vicinity of a wire supporting portion on the supply side of a wire transportation system, and a wire feed device is driven as the wire is guided to the delivery side by means of the water flow. This arrangement, however, requires a machining start hole greater in diameter than the water flow and a quantity of water large enough to guide the wire. Accordingly, it is difficult to apply this arrangement to automatic wire threading by means of a fine-diameter machining start hole.